1. Field of the Invention
The present invention relates to an ink jet type printer used for a recording apparatus such as a printer, a copier or facsimile to record characters and images. More particularly, the present invention relates to an ink jet head in which the deformation of a piezoelectric element is used for a drive source to drive an ink jet pump.
Recently, printers are indispensable for attaining office automation. Moreover, printers are coming widely into private use. As a printer for private use, the ink jet type printer attracts the attention of users because, compared with a wire drive type printer in which printing is conducted on a sheet of paper by pressing a wire against a platen via an ink ribbon, the ink jet type printer in which an ink jet head is used is advantageous in that: it makes little noise; the printing speed is relatively high; and the printing cost per sheet of paper is low.
2. Description of the Related Art
A pump to eject liquid by using the deformation of a piezoelectric element is characterized in that the speed of response is high. Therefore, it is possible to compose a pump having no valves. For the above reasons, the pump in which the deformation of a piezoelectric element is used is preferably applied to an ink jet head which must be compact and free of maintenance. When the passages for ink in the pump are designed in such a manner that the resistance and length on the input side are different from the resistance and length on the output side, ink is ejected onto the side of a short passage in the case of a quick contraction, and only a small quantity of ink is ejected onto the side of a long passage because of a delay in response. Successively, when a slow expansion is conducted in the pump, a difference is caused between a quantity of ink flowing into the short passage and that flowing into the long passage because the cross-sectional area of the short passage and that of the long passage are different from each other. Accordingly, it is possible to form a flow of ink which flows in one direction as a whole.
In the ink jet operation, when the pressure chamber is expanded, the outwards of a nozzle from which ink is ejected is exposed to the atmosphere. Therefore, atmospheric air is prevented from flowing into the pressure chamber via the nozzle by the action of surface tension generated on the atmosphere side of the nozzle. Accordingly, ink flows into the pressure chamber only from the feed passage, that is, the flow of ink has a strong directivity. As a body to drive the pump in this way in which a ratio of the ink flowing speed on the input side to that on the output side is high, a piezoelectric element is preferably used. Especially, the deformation of a piezoelectric element in the thickness direction can respond very quickly, that is, it can respond to a signal, the frequency of which is in the order of MHz, so that the piezoelectric element can be used as an ultrasonic oscillator.
In the case of a conventional ink jet head in which a piezoelectric element is used, only an expansion and contraction in the direction of polarity caused by the piezoelectric effect of the piezoelectric element is used as a means for pressurizing ink by changing a volume of the pressure chamber, or only an expansion and contraction in the perpendicular direction is used. Alternatively, in the conventional ink jet head, only a shearing deformation of the piezoelectric effect is used. In other words, only one direction of deformation caused by the piezoelectric effect is used as a means for pressurizing ink.
Concerning the deformation of a piezoelectric element, the direction of polarization is referred to as d.sub.33, and the direction perpendicular to the direction of polarization is referred to as d.sub.31. Concerning the deformation of a piezoelectric element, the piezoelectric constant d.sub.33 is 650.times.10.sup.-12 m/V at the most. Irrespective of the thickness of the piezoelectric element, even when the voltage is set at 100 V, an amount of expansion is approximately 70 nm. Accordingly, unless an area of the wall of the piezoelectric element to determine the pressure chamber is very large, the contracting volume of the pump is very small. That is, an amount of deformation is described as follows. When voltage E is impressed upon the piezoelectric element in the direction of polarization and the thickness of the piezoelectric element in that direction is t, the electric field intensity is E/t, and the deformation in the thickness direction (the direction of polarization) caused by that is expressed by the following equation. EQU (E/t).times.d.sub.33.times.t=Ed.sub.33
However, when the piezoelectric effect is provided by a piezoelectric element, an expansion and contraction and a shearing are caused in a plurality of directions. Accordingly, from the viewpoint of enhancing the efficiency of using energy, it is not advantageous that one of these deformations is used alone, but it is advantageous that the deformations are used while they are compounded with each other. When these deformations of a piezoelectric element are used while they are compounded with each other, even in the case of impressing a low voltage, it is possible to obtain a change in the volume of the pressure chamber, the amount of which is the same as that of the present ink jet head, and further the expense necessary for an electric power supply to drive the ink jet head can be reduced.
In view of the above circumstances, it is possible to compose an ink jet head of high energy efficiency when an expansion and contraction in the direction of d.sub.33, which is a direction of polarity provided by the piezoelectric effect of a piezoelectric element, is compounded with an expansion and contraction in the direction of d.sub.31, which is a direction perpendicular to the direction of polarity provided by the piezoelectric effect. When a voltage is impressed upon the piezoelectric element so as to contract the piezoelectric element in the direction of d.sub.31, the piezoelectric element is expanded in the direction of d.sub.33. Accordingly, one of these behaviors which are opposed to each other is utilized as a cause of the bimorph effect generated in a compound member composed of the piezoelectric element and a member made of material different from the material of the piezoelectric element. That is, ink is pressurized by superimposing a deformation caused by the piezoelectric effect of the piezoelectric element itself on a deformation caused by the bimorph effect generated in the compound member made of material different from material of the piezoelectric element.
When the plate-shaped piezoelectric element (activated section) is made to adhere to the unactivated section made of material different from material of the piezoelectric element, the piezoelectric element is expanded in the thickness direction, and at the same time it is contracted in the surface direction. Accordingly, a strain is caused between the piezoelectric element and the unactivated section. Due to the generation of strain, camber is caused so as to relieve the strain.
FIGS. 1 and 2 are views showing this concept. In FIGS. 1 and 2, reference numeral 1 is a compound plate-shaped member, reference numeral 2 is an activated section composed of a piezoelectric element, reference numeral 3 is an unactivated section, and reference numeral 4 is a stationary wall. The radius of curvature of the camber is larger on the unactivated section side (thickness h.sub.1) and smaller on the activated section side (thickness h.sub.2) which is located inside. Accordingly, the length of the unactivated section in the surface direction is a little different from the length of the activated section in the surface direction. Due to the foregoing, strain .delta. of the piezoelectric element is greatly changed. This principle has been applied to an ink jet mechanism for some time. However, application of this principle is restricted for the following several reasons.
(1) In order to increase a ratio of magnification, it is advantageous to use a thin piezoelectric element. However, it is difficult to form a thin piezoelectric element. PA1 (2) When the thickness of a piezoelectric element is reduced, it is possible to extend a strain volume. However, the rigidity is lowered, and it becomes impossible to generate a high pressure, that is, it is impossible to provide a sufficiently high ink ejecting pressure. PA1 (3) It is necessary to increase an activating area of the piezoelectric element. However, since restricted by the construction, only one surface of the piezoelectric element is driven. Therefore, ejection of ink deviates due to the unbalance of the activating area of the piezoelectric element.
On the other hand, in the case of an ultrasonic oscillator in which only a change in the thickness of the piezoelectric element is used, the rigidity is high, so that it is possible to generate a high pressure, however, a change in the volume is small. Accordingly, it is necessary to provide a wall of large area to be used as an activating surface.